Grids for lead-acid batteries provide structural support for the active material therein, and also serve as a current collector during discharge and current distributor during recharge. Accordingly, grid designs seek to optimize the amount of active material supportable by the grid to increase the current collection and distribution characteristics of the grid while minimizing the grid weight.
Known prior art grid designs, such as shown in FIGS. 1-3, include a top frame member 2 and a bottom frame member 3 joined by a plurality of metal wires 4 forming a pattern interposed between the frame members 2, 3. A lug 5 formed as an integral part of the top frame member 2 is interconnected with adjacent grids in a battery.
Known grid patterns include a diamond pattern, characterized by wires defining diamond shaped grid cells, such as shown in FIGS. 1 and 2, a rectangular pattern, characterized by rectangular grid cells, a radial pattern characterized by wires extending radially from a common point, such as shown in FIG. 3, and other grid patterns, such as disclosed in U.S. Pat. No. 5,582,936. These particular patterns have certain advantages and disadvantages which are discussed in further detail below.
Battery grids are commonly manufactured by processes, such as casting, expanded metal forming, and stamping. Cast battery grids are manufactured by pouring molten lead into a mold, allowing the lead to cool, and then separating the grid from the mold. The casting process is capable of producing a variety of efficient grid designs, which are limited only by the ability of mold makers to make the mold.
The casting process is, however, an expensive process which discourages its use. The process requires the use of a mold coating to facilitate separation of the grid from the mold, and for an increased throughput, a plurality of expensive molds are required. Furthermore, even with multiple molds, the casting process is still a batch process which tends to have a lower productivity (i.e., produces less product over a given time period) than a grid manufacturing process which is "continuous," such as expanded metal forming.
Grids formed from expanded metal are less expensive than molded grids because of the higher productivity of the expanded metal forming process over the casting process. In the expanded metal process, battery grids are formed by expanding metal through a process in which a strip of cast or wrought lead material is pierced and then pulled or expanded. In a conventional expanded metal grid, the grid mass is substantially evenly distributed across the grid, and the grid is limited in wire pattern, wire shape, and lead distribution.
Two particularly common expanded metal forming processes, rotary expansion and reciprocated expansion, have been developed. In the rotary expansion process, a lead strip is cut with a rotary cutter, the wires are extruded above and below the plane of the strip and then expanded in the horizontal directions to form a diamond grid pattern interposed between top and bottom frame members. In the reciprocated expansion process, wires defining a diamond grid pattern are cut and expanded in a direction perpendicular to a surface of the strip. After expansion, the strip is rotated 90.degree., and the grid is coined. The size of the diamond and the wire width are variables in either process.
The wire angle and wire size of an expanded metal grid pattern are limited to ensure proper expansion without breaking the wires. The wire angle, as shown in FIG. 1, is the angle A of the grid wires with respect to the top or bottom frame member 2, 3, and is typically less than 40.degree. in an expanded metal grid. This wire angle limitation creates a zigzag path for current to flow through the grid. The zig-zag pattern increases the grid resistance because the current does not flow directly to the collecting lug, such as in a radial grid formed by casting.
The wire size limitation also limits the taper rate to 15% or less for the rotary process, and 60% or less for the reciprocated process. The taper rate, best illustrated in FIG. 3, is the rate at which a wire width can be changed along its length. For example, with a 15% taper rate, the maximum wire width near the current collecting lug is 15% wider at the grid top than that at the grid bottom.
More lead mass in the lug area would enhance the current carrying capability of the grid and reduce the grid resistance because the current generated in a plate flows toward the lug. These features are difficult to achieve using the expansion process. Thus, the conductivity of expanded metal grids tend to be lower than a similar size cast grid.
Furthermore, there is no side frame in an expanded metal grid to restrict growth of the wires. Thus, the service life of an expanded metal grid is considerably shorter than the cast equivalent due to the upward growth of a positive expanded grid in a battery resulting in either shorting with an adjacent negative strap or loss of positive active materials.